Choices for the treatment for fractures of the long bones extend back over a long and complex history. The significant long bones of the extremities are the humerus, radius and ulna of the upper extremity and the femur and tibia of the lower extremity. The upper extremities are considered non-weight bearing while the lower extremities are weight bearing, i.e., normally bearing the weight of the individual. The length of these bones, in conjunction with their inter-articulations, forms the fundamental basis for an individual's ability to generate leverages, or moments of force, that enable the individual to walk, run, use tools and other heavy equipment, and, in general, interact physically with their environment.
At the moment of injury to bones, in particular, injuries resulting in fractures of the long bones, it is the inadvertent application of leverage-like forces over the length of a long bone that is the usual causative factor in fracturing. In general, the weight bearing bones are more difficult to heal and suffer more frequent complications associated with their injuries than the non-weight bearing bones. Additionally, it is well known in medicine that the earlier a patient is mobilized as part of a comprehensive rehabilitation program, the faster they heal, with stronger repair and less likelihood of associated complications. For weight bearing bones, early mobilization carries the added difficulty of providing for sufficiently stabilizing a fractured bone structure to carry weight before there has been adequate healing for the bone to bear that weight.
To reach this goal of early mobilization of the patient, a number of fixation devices, both external and implantable, have been devised over the centuries. These devices accomplish immobilization of the fracture fragments and stabilization of the fractured long bone, providing earlier mobility and weight bearing. Intramedullary (IM) nailing, particularly interlocking IM nailing, has become the standard procedure for immobilizing shaft fractures of long bones including proximal or distal fractures of the weight bearing long bones.
IM nails are available in any number of lengths, widths, cross-sectional shapes, with or without slots, and thickness of wall. A functional aspect common to all IM nails is the close association, a wedging so to speak, of the IM nail and the cortical bone inner wall of the isthmus of the intramedullary canal, also known as the contact area. The more longitudinal and circumferential the contact between the bone and the nail, the more stable the immobilization of the fracture fragments and the less motion that occurs at the fracture site.
Additional stability is achieved using a procedure known as interlocking. Whether an IM nail is also interlocked, and how the interlock is achieved, is dependent on the type of IM nail used, the type of fracture, and the experience of the operator. With this procedure, the IM nail is locked to the bone using either cortical screws or some sort of wing or anchor, depending on the method of interlock proximally, distally, or both. A dynamic lock is achieved when interlocking occurs only between the proximal end of the IM nail and the proximal portion of bone or just between the distal portion of the IM nail and the distal portion of the bone. Consequently, there is a definite potential for motion between the fracture pieces at the fracture site with weight bearing, but this procedure also provides for greater compression between the fracture pieces at the fracture site. Static interlocking involves interlocking the proximal and distal portions of the IM nail to those respective bone fragments, i.e., both the proximal and distal portions of the long bone are locked to the IM nail. With the IM nail bridging the fracture gap and fixated to the proximal and distal bone fragments, the probability of motion at the fracture site is markedly reduced. However, the ability to actively compress the fracture fragments into each other is also attenuated. Using newer compression techniques, the general consensus currently is to rely mostly on static interlocking.
The most common form of interlocking is the cortical bone lock. This procedure uses cortical screws placed through the bone and IM nail, transverse or oblique to the long axis of the bone and IM nail. The screws are first driven through the near wall of bone, then guided through pre-formed holes in the near and far walls of the IM nail, and finished by imbedding into the opposite cortical bone wall. Another form of interlocking is the cancellous bone lock. This interlock uses deployable wings, or anchors, pre-attached to the IM nail, that are deployed into the cancellous bone of the marrow space after the IM nail is positioned. These locks are subcortical in the cancellous bone and because of the loose trabecular nature of cancellous bone, are considerably less stable than cortical locking. For additional discussion concerning IM nails and interlocking, see Bechtold, Joan E., Ph.D., Chapter 2: Biomechanics of Fracture Fixation Devices in Fractures and Dislocations, volume 1 Gustilo, Ramon B., M.D., Kyle, Richard F. M.D., and Templeman, David C. M.D., editors (Mosby-Year Book, Inc. 1993).
A significant disadvantage to the procedure for cortical interlocking IM nails is placement of the cortical screws through the pre-formed holes of the IM nail. The major difficulty encountered is targeting the distal screw holes. Proximal screw holes are usually easier to locate, particularly with the use of targeting jigs. As the IM nail is inserted into the intramedullary canal, it can twist and deform, therefore, the surgeon cannot readily rely on proximally based targeting jigs for the placement of the locking screws through the distal holes.
Therefore, the generally accepted technique currently is to use x-rays and an x-ray image intensifier built into a C-arm apparatus. The image is provided through a fluoroscopic video in real time. The C-arm is positioned so that the central axis of the x-ray bean is collinear with the central axis of the distal screw holes. This shows up as aligned screw holes without parallax. An incision is then made over the position of the IM nail holes and the bone surface is marked on the path of the alignment. A small hole is usually drilled freehand into the near wall of bone to reach the near side pre-formed hole in the IM nail with the purpose of confirming accuracy. If placement is adequate, the hole in the bone is widened sufficiently to accommodate a screw that is inserted through the widened hole and ultimately embedded within the cortical bone.
Using x-rays, while relatively easy, does have complications and significant drawbacks. One such drawback is that the cumulative dose of x-ray radiation to the surgeon and other health care providers may become large over time. Another is that the technique is not foolproof, erratic screw placement occurs as well as missing distal holes entirely. If a hole is missed, more drilling is required, usually in a new site or by widening the first hole, both of which weaken the bone. Another drawback is the cost and availability of the C-arm and imaging apparatus making this a cost prohibitive procedure in under-developed countries.
There exists a distinct need for an interlocking IM nail and method that does not need to rely on x-ray imaging, extraneous alignment jigs or pre-formed screw holes. An improved IM nail and method would continue to use cortical screws in either a dynamic or a static interlocking IM nail fixation.
There is a need for an intramedullary nail device that will substantially reduce, if not eliminate many of the difficulties encountered with current intramedullary nails and other fixation systems. Such a device and method would provide appropriate fixation and stabilization without the need for x-ray radiation methods of alignment, eliminating the risk of excess exposure to x-ray radiation, without excess loss of good bone stock, that is easy to implant and interlock, and is substantially cheaper to manufacture.